1. Field of the Invention
The present invention relates to an Internet protocol (IP) telephony system and, more particularly, to a gatekeeper supporting a handoff and handoff method in an IP telephony system.
2. Background of the Related Art
With an increase in the demand of terminals in the LAN environment, mobility support for an IP terminal is expected to heighten productivity of IP telephony.
Currently, two institutions standardize an IP telephony protocol. These are an International Telecommunication Union (ITU), which has adopted H.323 as a standard, and an Internet Engineering Task Force (IETF), which has adopted Session Initiation Protocol (SIP) as a standard. The H.323 standard has been widely implemented as a product, defining a system component and a control message or the like for a multimedia communication in a packet-based network.
In order to support mobility of a terminal, an Annex H of H.323 has been proposed as a draft. H.323 Annex H proposes to support a handoff using a mobile IP and H.323 ad hoc conference signaling. H.323 ad hoc conference signaling necessarily requires a multipoint controller (MC). The MC performs a control function and terminals attending the ad hoc conference are to react suitably to various command messages transmitted through a H.245 control channel
When a gatekeeper receives an ad hoc expansion signaling request, it is operated in an MC mode. The gatekeeper transmits a message informing the conferencing terminals that a corresponding call is extended to an ad hoc conference mode and then the terminals respond to the message transmitted from the MC, thereby performing the ad hoc conference signaling.
A conventional handoff method using the ad hoc conference signaling proposed by H.323 Annex H will next be described. H.323 Annex H is a draft for supporting mobility of the terminal and defining a home gatekeeper and a foreign gatekeeper as concepts equivalent to a public land mobile network (PLMN) and a home location register (HLR). A home zone is managed by the home gatekeeper, and is where a mobile terminal remains most of the time without movement. A foreign zone is managed by the foreign gatekeeper, to where the mobile terminal is movable. Movement of the mobile terminal between subnets in a zone is called an intra-zone roaming and movement of the mobile terminal between zones is called an inter-zone roaming.
H.323 Annex H views a handoff in a concept that the mobile terminal dynamically participates in the ad hoc conference, regardless of the intra-zone roaming and the inter-zone roaming.
FIG. 1 is a flow chart of a related art handoff using the ad hoc conference signaling proposed by H.323 Annex H. A mobile H.323 terminal (MT1) provides an IP telephony service to a subscriber according to the H.323 protocol. A fixed H.323 terminal (FT) provides the IP telephony service to the subscriber according to a H.323 terminal. The gatekeeper (GK) performs a function of registering and managing information of the H.323 service subscriber, a function of call admission and authorization, an address conversion function, and a zone management function. For the purpose of describing FIG. 1, it is assumed that MT1 is an origination terminal, FT is a destination terminal, MT1 and FT are in a connected state, and the MT1 terminal is re-designated as an MT2 terminal after moving to a different subnet.
As shown in FIG. 1, when a terminal MT1/MT2 roams to another subnet, it discovers that its own network connection point has been changed through a mobile IP, a gatekeeper request (GRQ), and a mobility gatekeeper advertisement (MGA) message (step S11). MT2 obtains a care of address (COA), which is a new IP address, and is admitted for registration to the gatekeeper with the obtained IP address as its own IP (steps S12˜S15). After the terminal is registered, the terminal transmits a set-up (goal=JOIN) message to the call with which it was related before roaming (step S16). This message induces an ad hoc conference signaling so that a media channel is rerouted to the terminal to thereby support a handoff.
The H.323 ad hoc conference signaling needs the MC, which controls the conference signaling, and the terminals attending the ad hoc conference are to react properly to the MC Location Indication, Terminal Number Assign, Communication Mode Command messages transmitted from the MC of the GK.
Upon receipt of the set-up (goal=JOIN) message from MT2, the gatekeeper performs a mode switch for related call information from a peer-to-peer call to the multipoint call. While performing the mode switch, the gatekeeper performs the MC function.
The gatekeeper does not transmit the SETUP message received from MT2, in the MC mode, to FT or MT1. However, the gatekeeper transmits a CONNECT message directly to MT2 (steps S17, S18). Upon receipt of the CONNECT message, MT2 and the gatekeeper perform a control channel set signaling and up to a master slave determination signaling. Until then, corresponding H.245 messages are transmitted and received only between the gatekeeper and MT2.
When the master slave determination is performed, the gatekeeper sends an MC Location Indication message to inform the terminals attending the ad hoc conference that the corresponding call has been changed to the multi-point call (step S19). Additionally, the GK sends the terminals a terminal number assignment message (step S20). In addition, to inform the conference participants that the ad hoc conference has been selected as a common mode, the gatekeeper transmits a Communication Mode Command message to the terminals attending the ad hoc conference (step S21).
If a currently opened real-time transport protocol (RTP) mode is different from the mode of the Communication Mode Command message, the terminals which receive the corresponding message should have a function to transmit a logical channel setup message to the gatekeeper and transmit a logical channel release message to the gatekeeper (step S22).
An environment in which the gatekeeper supports only the MC and does not support the multipoint processor (NP), which mixes several media streams, brings about more complicated signaling. That is, every terminal attending the ad hoc conference should multicast its own audio data and mix every audio stream coming to itself. Accordingly, the terminal performs the logic channel set-up message several times.
In addition, as shown in FIG. 1, since MT1 does not exist at the present time, FT and MT2 are to process the logic channel set-up message in consideration of MT1. A handoff signal using the ad hoc conference signaling is to be performed differently to the general ad hoc conference signaling. Thus, in a case where a handoff is supported through the ad hoc conference signaling, the terminal is to process so many works and a long signaling time is taken due to many message exchanges.
On the other hand, the H.323 ad hoc conference signaling used for the handoff can be replaced with call transfer signaling. The call transfer is a function of turning a call for someone to a different user, which, thus, can be used for a handoff signaling that turns a call to a different area where a mobile terminal has been moved.
In H.323, the call transfer function is implemented through H.450.1, H.450.2. Numerous options and error conditions require this protocol for a supplementary service, which can be implemented in a H.323 terminal and a gatekeeper.
H.323 is disadvantageous in that its call set-up time is longer than that of the SIP, because, despite a H.323 peer-to-peer call, it is performed through five steps of an admission Request/Admission confirm (ARQ/ACF) round trip, a SETUP/CONNECT round trip, a H.245 capability set exchange round trip, a H.245 master slave determination round trip, and a logical channel setup round trip.
In addition, the Q.931 channel used for the SETUP/CONNECT round trip and the H.245 call control channel used for the H.245 incoming/outgoing capability exchange and H.245 master slave determination round trip are made on the basis of a transmission control protocol (TCP). Q.931 is a call signal protocol and H.245 is a control protocol for multimedia communication. The TCP connection needs an additional delay for synchronization of a TCP window sequence number, which may take hundreds of milliseconds (ms) in a wide area network (WAN) environment.
In order to make up for the shortcomings, H.323 v uses fast connect and tunneling. Fast connect finishes a call set-up with a SETUP/CONNECT procedure. Tunneling causes the H.245 call control channel and the Q.931 channel to use the same TCP connection, so as to provide a prompt call set-up time.
However, a call made by the fast connect is not obliged to open the H.245 control, according to the ITU the recommendation. Thus, the existing gatekeeper does not open the H.245 control channel when a call is established by the fast connect. Thus, a third party initiated pause and rerouting function using the H.245 control channel is not provided, so that H.450.2 should be implemented in both the gatekeeper and the terminal for the service using the call transfer. H.450.2 is a H.323 supplementary service protocol.
The related art handoff method using the call transfer will now be described.
FIG. 2 is a flow chart of a handoff signaling method using a call transfer in case of a call without a control channel for a multimedia communication, that is, without a H.245 control channel. A call has been connected between MT1 and FT. MT2 is the new designation for terminal MT1 after MT1 has moved to another subnet. FT and MT1 set up a call by the fast connect and transmit and receive data (steps S31, S32). Thereafter, when MT1 is re-designated as MT2 and MT2 requests a registration from the gatekeeper, the gatekeeper admits a registration of MT2 (steps S33, S34).
Since there is no control channel for a multimedia communication in the corresponding call, the gatekeeper, which has admitted registration of MT2, carries a CT_INIT.INVOKE Application Protocol Data Unit (APDU) informing FT that MT1 has been re-designated as MT2, on a FACILITY message, and transmits it to FT (step S35). Upon receipt of the FACILITY message, A(FT) analyzes the CT_INIT.INVOKE and transmits a SETUP message to the gatekeeper. At this time, FT carries a CT_SETUP.INVOKE APDU on the SETUP message to inform the gatekeeper that the corresponding call is to be performed as a call transfer.
Upon receipt of the SETUP message, the gatekeeper analyzes the CT_SETUP.INVOKE APDU of the SETUP message, records it in call set-up information, and transmits the SETUP message to MT2. When the gatekeeper transmits the SETUP message, it reproduces the CT_SETUP.INVOKE APDU using the information analyzed from the call set-up information and transmits it to MT2 (step S36).
Upon receipt of the SETUP message, MT2 carries a CT_SETUP.RRAPDU on a CONNECT message and transmits it to the gatekeeper. Then, the gatekeeper transmits the received CONNECT message to the A(FT), so that the call transfer is performed (step S37). Thereafter, the A(FT) sends a RELOCM message to B(MT1) (step S38).
In detail, in the call transfer structure as shown in FIG. 2, a supplementary service stack, that is, a H.450.2 stack, is requested to be wholly implemented in the terminal. The reason for this is that the gatekeeper generates, or the terminals which transmits the H.450 APDU generate, a timer for the corresponding APDU and processes the abnormal state when a result value is not received within a predetermined time.
Thus, the terminals should analyze every H.450.2 ADPU and record the control state of the corresponding message. In this respect, however, it is very difficult to implement the H.450.2 stack with every case considered, since an IP phone has a memory restriction.
In addition, the method where H.450 stack is implemented in both the gatekeeper and the H.323 terminal, to support a supplementary service in a distributed form, makes the signaling time long due to the increase of exchanged messages.
Thus, the handoff using the call transfer in a distributed form, not the gatekeeper leading form, increases a burden to the terminal like the H.323 ad hoc conference signaling, causing the problem that the handoff signaling time is lengthened.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.